1. Field of the Invention
This invention relates to a small-sized centrifugal pump suitable for an electric motor-driven washer pump for supplying a washing agent in order to clean a windshield or a head lamp of motor vehicles and, more particularly to a small-sized centrifugal pump suitable to obtain excellent pump performance with high efficiency in a centrifugal pump of the low-specific speed type.
2. Description of the Prior Art
In the case of cleaning a windshield or a head lamp of motor vehicles by spraying a washing agent supplied by using an electric motor-driven washer pump, it is necessary to clean efficiently with a small amount of the washing agent since the washing agent is stored in a reservoir and is limited in quantity. For example, a washing agent having a flow rate of about 280 cc/10 sec is sprayed at a pressure of about 1.5 kgf/cm.sup.2 in the case of a windshield washer pump, and the a washing agent having a flow rate of about 1000 cc/10 sec is sprayed at a high pressure of about 3.0 kgf/cm.sup.2 in the case of a head lamp cleaner pump, and in either case the specific speed of the pump is so low as to be less than 80.
Presently, a centrifugal pump, a vane pump and a gear pump are cited as the type of pump used for the washer pump. Among them, the centrifugal pump is used generally except for one used under special conditions. The reason why the centrifugal pump is used is that it can be produced easily at a low price, and it is not so noisy because it is not a contact type. However, in the centrifugal pump, it is necessary to design an impeller to lower its height against the diameter of the impeller in order to decrease the specific speed of the pump. Therefore, a flow channel in the impeller becomes flat, and the efficiency of the pump drops drastically due to a loss due to circulation in the flow channel or a loss due to separation caused at the surface of a blade of the impeller.
Especially, in case of the washer pump having an impeller with a diameter of 30 mm or so, exit height h.sub.2 (height at the circumference) of an impeller 51 shown in FIG. 11(a) is calculated to be not more than 1 mm according to theoretical calculations. Accordingly, a leakage loss in the pump housing becomes serious in addition to the deterioration of the performance because a ratio (h.sub.2 /c) of the exit height h.sub.2 of the impeller 51 to a clearance c of a pump housing, and the desired high performance can not be obtained. Such a tendency is more remarkable the move since the size of the washer pump is small.
Therefore, in the conventional washer pump, the impeller 51 is designed so that height H of the impeller 51 is 4.about.6 mm or so, as shown in FIG. 11(b), and the pump intentionally has a specific speed which is set higher than the theoretical calculation. In such a case, it is possible to obtain the good performance, but the losses caused by the separation and the circulation are remarkable and it is impossible to obtain the high efficiency because the capacity of the impeller (volume of the flow channel of the impeller) is larger than the theoretical value. Namely, the pump efficiency is of secondary importance, and obtaining the required performance of the pump is prior to the pump efficiency.
For example, a pump designed by optimizing the impeller diameter and the position of an outlet hole of the pump in order to improve the pump efficiency is disclosed in Japanese Patent Disclosure (Kokai) No. 62-15011/87. However, the method is merely a means for converting the velocity energy given by the impeller into the pressure energy effectively. Therefore, it is not a radical measure to obtain the highly efficient pump because the efficiency of the impeller itself is not improved.
In the conventional impeller 51, the fluid does not flow along the blade, the distribution of pressure in the flow channel becomes uniform and the separation and the circulation occur because the flow channel between the blades is spread outwardly from the center portion as shown in FIG. 11 (c), and the loss in the impeller is caused by these phenomena. There is also a problem in that the deterioration of the efficiency is caused by rotating the excessive fluid wastefully in the impeller 51 because the impeller 51 has a shape suitable for higher specific speed (a shape having a higher height for the impeller) and the volume of the flow channel in the impeller 51 is larger than necessity.